This invention relates to a radar seeker of the kind employing an array of antenna elements disposed about the boresight in such manner that difference signals between the elements can be obtained in two coordinate planes, usually the elevation and azimuth planes. Such a difference signal, together with a sum signal derived from all of the elements, provides a measure of the target angle off-boresight in known manner. The difference signal may be the difference between the individual amplitudes, in an amplitude comparison system using antenna elements toe'd out from the boresight, or may be the instantaneous difference, in a phase comparison system using antenna elements directed parallel to the boresight.
FIG. 1 shows a typical prior art antenna array with elements A, B, C and D arranged in a diamond formation, i.e. a square with horizontal and vertical diagonals. An elevation difference is then provided by a difference signal A-B and an azimuth difference by a difference signal C-D. In an alternative arrangement the square may be upright and the element signals added and subtracted in pairs.
FIG. 2 shows the basic components of a prior art amplitude comparison system, i.e. four elements A, B, C and D, a receiver in each element channel comprising a frequency mixer to `down-convert` the radar frequency to an intermediate frequency and an envelope detector. The A and B receiver channels are applied to a differencing circuit 11 and the C and D receiver channels similarly to a differencing circuit 13. Thus elevation and azimuth difference signals are produced. A summing circuit 15 derives the sum of all of the signals, this sum being used as a reference for the difference signals in a processor 17 which calculates the target azimuth and elevation angles and provides tracking and missile steering signals accordingly.
FIG. 3 shows a similar prior art arrangement but of a phase comparison system. Intermediate frequency signals are derived by `down-converters` 19 but the phase is left intact for application to phase-sensitive detectors 21 and 23 which produce a signal proportional to the cosine of the phase difference. A sum signal is provided by a summing circuit 25 as before, now followed by an envelope detector 27. The sum and difference signals are then processed by processor 17 as before.
A difficulty arises in the above arrangements in that considerable processing of the individual antenna signals occurs in separate channels which may not be perfectly matched in gain or phase or both. Angle determination errors consequently arise.